gluTessCallback is used to indicate a callback to be used by
a tessellation object. If the specified callback is already defined, then
it is replaced. If CallBackFunc is NULL, then the existing callback becomes
undefined.

These callbacks are used by the tessellation object to describe
how a polygon specified by the user is broken into triangles. Note that
there are two versions of each callback: one with user-specified polygon
data and one without. If both versions of a particular callback are specified,
then the callback with user-specified polygon data will be used. Note that
the polygon_data parameter used by some of the functions is a copy of the
pointer that was specified when gluTessBeginPolygon was called. The legal
callbacks are as follows:

GLU_TESS_BEGIN

The begin callback is invoked
like glBegin to indicate the start of a (triangle) primitive. The function
takes a single argument of type GLenum. If the GLU_TESS_BOUNDARY_ONLY property
is set to GL_FALSE, then the argument is set to either GL_TRIANGLE_FAN,
GL_TRIANGLE_STRIP, or GL_TRIANGLES. If the GLU_TESS_BOUNDARY_ONLY property
is set to GL_TRUE, then the argument will be set to GL_LINE_LOOP. The function
prototype for this callback is:

void begin ( GLenum type );

GLU_TESS_BEGIN_DATA

The same as the GLU_TESS_BEGIN callback except that it takes an additional
pointer argument. This pointer is identical to the opaque pointer provided
when gluTessBeginPolygon was called. The function prototype for this callback
is:

void beginData ( GLenum type, void *polygon_data );

GLU_TESS_EDGE_FLAG

The edge flag callback is similar to glEdgeFlag. The function takes a single
boolean flag that indicates which edges lie on the polygon boundary. If
the flag is GL_TRUE, then each vertex that follows begins an edge that
lies on the polygon boundary, that is, an edge that separates an interior
region from an exterior one. If the flag is GL_FALSE, then each vertex that
follows begins an edge that lies in the polygon interior. The edge flag
callback (if defined) is invoked before the first vertex callback.

Since
triangle fans and triangle strips do not support edge flags, the begin

callback is not called with GL_TRIANGLE_FAN or GL_TRIANGLE_STRIP if a
non-NULL edge flag callback is provided. (If the callback is initialized
to NULL, there is no impact on performance). Instead, the fans and strips
are converted to independent triangles. The function prototype for this
callback is:

void edgeFlag ( GLboolean flag );

GLU_TESS_EDGE_FLAG_DATA

The same as the GLU_TESS_EDGE_FLAG callback except that it takes an additional
pointer argument. This pointer is identical to the opaque pointer provided
when gluTessBeginPolygon was called. The function prototype for this callback
is:

void edgeFlagData ( GLboolean flag, void *polygon_data );

GLU_TESS_VERTEX

The vertex callback is invoked between the begin and end callbacks. It is
similar to glVertex, and it defines the vertices of the triangles created
by the tessellation process. The function takes a pointer as its only argument.
This pointer is identical to the opaque pointer provided by the user when
the vertex was described (see gluTessVertex). The function prototype for
this callback is:

void vertex ( void *vertex_data );

GLU_TESS_VERTEX_DATA

The same as the GLU_TESS_VERTEX callback except that it takes an additional
pointer argument. This pointer is identical to the opaque pointer provided
when gluTessBeginPolygon was called. The function prototype for this callback
is:

void vertexData ( void *vertex_data, void *polygon_data );

GLU_TESS_END

The end callback serves the same purpose as glEnd. It indicates the end
of a primitive and it takes no arguments. The function prototype for this
callback is:

void end ( void );

GLU_TESS_END_DATA

The same as the GLU_TESS_END
callback except that it takes an additional pointer argument. This pointer
is identical to the opaque pointer provided when gluTessBeginPolygon was
called. The function prototype for this callback is:

void endData ( void
*polygon_data);

GLU_TESS_COMBINE

The combine callback is called to create
a new vertex when the tessellation detects an intersection, or wishes to
merge features. The function takes four arguments: an array of three elements
each of type GLdouble, an array of four pointers, an array of four elements
each of type GLfloat, and a pointer to a pointer. The prototype is:

The vertex is defined as a linear combination of up to four existing
vertices,

stored in vertex_data. The coefficients of the linear combination
are given by weight; these weights always add up to 1. All vertex pointers
are valid even when some of the weights are 0. coords gives the location
of the new vertex.

The user must allocate another vertex, interpolate parameters
using

vertex_data and weight, and return the new vertex pointer in outData.
This handle is supplied during rendering callbacks. The user is responsible
for freeing the memory some time after gluTessEndPolygon is called.

For
example, if the polygon lies in an arbitrary plane in 3-space,

and a color
is associated with each vertex, the GLU_TESS_COMBINE callback might look
like this:

If the tessellation detects an intersection, then the GLU_TESS_COMBINE
or

GLU_TESS_COMBINE_DATA callback (see below) must be defined, and it
must write a non-NULL pointer into dataOut. Otherwise the GLU_TESS_NEED_COMBINE_CALLBACK
error occurs, and no output is generated.

GLU_TESS_COMBINE_DATA

The same
as the GLU_TESS_COMBINE callback except that it takes an additional pointer
argument. This pointer is identical to the opaque pointer provided when
gluTessBeginPolygon was called. The function prototype for this callback
is:

The error callback is called when an error is encountered.
The one argument is of type GLenum; it indicates the specific error that
occurred and will be set to one of GLU_TESS_MISSING_BEGIN_POLYGON, GLU_TESS_MISSING_END_POLYGON,
GLU_TESS_MISSING_BEGIN_CONTOUR, GLU_TESS_MISSING_END_CONTOUR, GLU_TESS_COORD_TOO_LARGE,
GLU_TESS_NEED_COMBINE_CALLBACK or GLU_OUT_OF_MEMORY. Character strings describing
these errors can be retrieved with the gluErrorString call. The function
prototype for this callback is:

void error ( GLenum errno );

The GLU
library will recover from the first four

errors by inserting the missing
call(s). GLU_TESS_COORD_TOO_LARGE indicates that some vertex coordinate
exceeded the predefined constant GLU_TESS_MAX_COORD in absolute value,
and that the value has been clamped. (Coordinate values must be small enough
so that two can be multiplied together without overflow.) GLU_TESS_NEED_COMBINE_CALLBACK
indicates that the tessellation detected an intersection between two edges
in the input data, and the GLU_TESS_COMBINE or GLU_TESS_COMBINE_DATA callback
was not provided. No output is generated. GLU_OUT_OF_MEMORY indicates that
there is not enough memory so no output is generated.

GLU_TESS_ERROR_DATA

The same as the GLU_TESS_ERROR callback except that it takes an additional
pointer argument. This pointer is identical to the opaque pointer provided
when gluTessBeginPolygon was called. The function prototype for this callback
is: